Undelivered Mail Returned to Sender include both success and fail email

Question

Dear All, 
 
 I am testing the policy for sending to special recipient like sending to user1 is allow but deny for user2. When I test sending to the user seperate, the result is acceptable. Email send to user1 is success and no 'Undelivered Mail Returned to Sender' notice send back to me. And user2 cannot receive the email and a 'Undeliverd Mail Returned' notice send back to me and tell me that user2 was reject for policy reason. 
 Howerver, when I include both user1 and user2 in the recipients, the email send to user1 and not to user2. It was the result I want. And a 'Undelivered Mail Returned to Sender' will send back to me. I expected the content will tell me user2 was rejected. However, it will includes both user1 and user2 in the content and said 
 user1:host 127.0.0.1[127.0.0.1] said: 550 This message was rejected for policy reasons 
 user2:host 127.0.0.1[127.0.0.1] said: 550 This message was rejected for policy reasons 
 So I am not sure is user1 recevied the email or not by the 'Undelivered Mail Returned to Sender' notice.

Red_Warrior · Answer

when the local loopback device rejects mail 127.0.0.1 that is postfix delivering the message to the milter process (the policy engine) 
 if you include multiple recipients then the message will essentially be split for each recipient, policy is run against each copy of the message. 
 for example. 
 1 message, 3 recipient's john jim and jane get a message, mailed from mail chimp (aka a bulk mail flyer) 
 
 the message is received and split 3 times.. the applaince will run all 3 copies against each users allow/block list and then against the UI configured policy. 
 So if John whitelists the sender, Jim blacklists it and Jane is omitted from the bulk mail rule in the applaince configuration.. Then 1 copy of the message is sent to John because he whitelisted the mailer.. Jims copy is destroyed because he blacklisted the mailer and Jane would get the list because she is omitted from the bulk mail rule.